Obesity causes dysfunction in major metabolic tissues. The glucose-fatty acid cycle, also known as the Randle hypothesis, provides the first basic concept for how elevated lipid levels in adiposity can cause dysregulated glucose metabolism and insulin resistance. Lipotoxicity is believed to be due to lipid overflow from adipocytes. Lipidomics studies in humans demonstrate that the lipid composition of serum is more closely resembles that of the liver, rather than white adipose tissue. Furthermore, the rhythmicity of hepatic metabolic function is known to couple the feeding behavior to biochemical reactions of substrate usage, suggesting that hepatic de novo lipogenesis also plays an important role in metabolic homeostasis. In the previous funding period, we proposed to examine the function of the nuclear receptor PPAR? in hepatic de novo lipogenesis. We find the expression of PPAR? and lipogenic genes, such as FAS, ACC1, ACC2 and SCD1, in mouse liver is under circadian regulation, being active in the dark (feeding) cycle. In liver-specific PPAR? knockout (LPPARDKO) mice, the up-regulation of ACC1 in the dark cycle is abolished, whereas the circadian pattern of FAS, ACC2 and SCD1 is shifted. Interestingly, the circadian rhythm of the hepatic lipogenic program correlates with the daily cycling of muscle fatty acid uptake/utilization, which is attenuated in LPPARDKO mice. Using unbiased, liquid chromatography-mass spectrometry (LC-MS) metabolomics approaches, we have identified 1-stearoyl-2- oleoyl-sn-glycero-3-phosphocholine (18:0-18:1 PC or SOPC), a phospholipid associated with PPAR?-regulated lipogenic program whose serum concentration peaks in the dark cycle. SOPC injection in mice promotes hepatic lipid synthesis/output and muscle FA utilization. In the renewal proposal, we will test the hypothesis that PPAR?-regulated lipogenic program modulates liver metabolism and muscle fatty acid utilization, in part, though SOPC. This research plan is innovative, as it describes a mechanism through which the circadian clock aligns the metabolic processes of feeding to peripheral energy substrate utilization. It is also on of the first studies, which use unbiased approaches to examine metabolomes using multiple mouse genetic models of metabolic diseases. The study is expected to lead to new areas of research and to have important clinical implication, as this novel lipid signaling may serve as a marker and a therapeutic target of metabolic diseases.